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About to drop a road flare down the gas tank

J

Jakal3482

NAXJA Forum User
Location
Battle ground, wa
I have a 90 cherokee. I bought it and the guy had just rebuilt the engine. Basically what it does.it falls on its face when you take off from a dead stop, and when in neutral it backfires through the intake.

I have tested the tps, map, cps, iac, cleaned the throttle body, tune up with champion plugs, and no change.

The Strange thing is when I unplug the map sensor vacuum line it doesn't do it, just idles like crap.

I dont think the bendix brakes are tied into the system at all. And off the intake manifold it holds 14 psi vacuum at idle with no fluctuation.

With the map plugged in at idle I'm at 15° on the timing mark, I unplug the vacuum line and it drops to 8°.

Maybe I'm going about It all wrong. I'm not sure where to look. I tried to smove test for leaks and got nothing.

My egr is deleted and everything is capped off, or plugged.

Any help is greatly appreciated.
 
Backfiring through the intake is either bad timing or a burnt valve.

It appears you looked at the timing. 15 degrees is about right at an idle.

Easiest is to do a compression test and while you are at it take a look at the plugs. Post up pix if you can.

Normally a non fluctuating vacuum would say your intakes are fine, but you never know. Your vacuum is a bit low.

Are you absolutely sure the plug wires are in the right order?
 
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I once experienced backfiring through the intake and air box and ended up being a clogged cat not allowing the exhaust to get out through the exhaust tip so it would take the path of least resistance back up through the intake.
 
Cruiser54 has a great write up for indexing the distributer on a Renix. Used it myself when I bought my last rig and realized the dizzy was off a tooth and it made it easy for a job that I was always nervous about doing.
 
Here is his write up on it:

Remove the distributor cap and cut a "window" into the side of the distributor cap at the #1 spark plug wire post . The "window" should be large enough to allow easy visual inspection of the position of the distributor rotor at the #1 spark plug wire post. Reinstall the distributor cap.

Install a ¾" wrench or socket onto the vibration damper retaining bolt. Rotate the engine in a clockwise direction until the #1 cylinder is at top dead center. Align the timing mark on the vibration damper with the "0" degree mark on the front cover timing scale. The tip of the distributor rotor should be near the #1 spark plug wire post.

Disconnect the distributor electrical connection. Remove the distributor holddown clamp, holddown bolt and distributor. Remove the distributor cap and rotor.

Place the distributor housing upside down in a soft jaw vise. Scribe a line 1/2 inch from the end of the distributor locating tab. Cut the distributor locating tab at the scribed line with a saw.

Remove any burrs and metal filings from the distributor. Reinstall rotor.

If necessary, using a flat blade screwdriver, turn the oil pump gear drive shaft until the slot is slightly past the 11 o'clock position. The oil pump gear drive shaft is accessible through the distributor mounting bore in the engine block.

Visually align the modified locating tab area of the distributor housing with the holddown clamp bolt hole.

Turn the rotor to the 4 o'clock position.
Lower the distributor into the engine block until it seats. The rotor should now be very close to the 5 o'clock position.

Reinstall the distributor cap with the cutout "window". Rotate the distributor housing until the trailing edge of the distributor rotor tip is just departing from the #1 spark plug wire post terminal .

Reinstall the distributor hold-down clamp and bolt.. Reinspect the position of the rotor to the #1 spark plug wire post to insure that it has not moved.

Install the new distributor cap, reconnect the distributor electrical connections.
Renix20dizzy20indexing_zpsjw3mj7dg.jpg
 
I did check the timing, did all but cut the tabs off. Mine looks just like that pic. My plugs actually looked almost carbon fouled after letting it idle for 20 min. I would believe the burnt valve idea, but when I unplug the map sensor it revs like a brand new truck. Plug wires are correct.
 
Have you actually replaced the MAP sensor or just tested it? What about the wiring going to it? Maybe some issues with a break in the wiring could be causing bad readings.
 
Just make sure when installing it you keep it looking new and take it back if it doesn't solve your problem. You could also trace the ground for that sensor to make sure it's grounding block/strap is secure.
 
Good point. I read about just adding a ground for the map. Then other thing I need to do us add a ground to the dipstick bolt. I guess all the major sensors ground there. I feel like it's around the map since that's the only one that changes the way it runs when I mess with it.
 
While you're there, rent out a compression tester and do a compression test like Old_Man suggested too. Will tell you a lot about the status of your engine.
 
If you haven't blown it up already have you tested fuel pressure or replaced filter?
 
Fuel pressure was 33, and 38 unplugged. Haven't changed the filter, tho I have one for it. I don't feel like that's the problem since it runs just fine with the map unplugged. I think if it were pump or filter related it would always do it. Right?
 
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